Plasma arc torches as known in the prior art are capable of efficiently converting electrical energy to heat energy at very high levels of temperature. It is not uncommon for a plasma arc torch to operate in a range of 6000.degree.-7000.degree. C.
Plasma arc torches are typically of two types, one of which can be operated in a transfer arc mode and the other of which can be operated in a non-transfer arc mode. In the transfer arc mode, the electric arc jumps from a first electrical terminal on the torch to a second terminal which is essentially the workpiece, i.e. the arc connects, or transfers to, the workpiece. In the non-transfer arc mode, the arc jumps from a first terminal on the torch to a second terminal which is also on the torch, thereby not transferring from the torch.
Prior art plasma arc torches are typically made to operate in either a transfer configuration or a non-transfer configuration. More recently, a convertible torch has been described in U.S. Pat. No. 4,549,065 to Camacho et. al. entitled PLASMA GENERATOR AND METHOD and more specifically in U.S. Pat. No. 4,559,439 to Camacho et. al. entitled FIELD CONVERTIBLE PLASMA GENERATOR AND ITS METHOD OF OPERATION, the teachings of both of which patents are incorporated herein by reference. The '439 patent describes a torch having mounting means adapted to threadably mount either an electrode assembly for operation in the non-transfer mode or a collimating nozzle assembly for operation in the transfer mode.
The convertible plasma torch of the '439 patent requires that the entire front electrode assembly be removed and replaced by the collimating nozzle assembly when the torch is being converted from a non-transfer to a transfer mode. The present invention recognizes that significant advantages would accrue if certain of the components making up the front electrode assembly could be used as part of the collimating nozzle assembly. Thus, an object of the present invention is to provide an improved convertible plasma torch in which certain components making up the front electrode assembly can be used as part of a collimating nozzle assembly.
In another aspect of the prior art evolving from the cup shape front electrode as illustrated in the '439 patent, it has been observed that such cup shape geometry promotes the arc attachment in such manner that erosion of the front electrode takes place primarily in the cup region. Thus, another object of the present invention is to design the internal shape of the front electrode in such a manner that the wear pattern will automatically create an expansion region that promotes a natural axial erosion of the electrode.
The invention disclosed by the description and the drawings to follow relates to an additional and novel apparatus and method for simply converting a plasma arc torch from one mode of operation to the other.